ABSTRACT Opioid use disorder (OUD) is a major epidemic in the US. Drug craving and relapse are major challenges in treating OUD as abstinent drug users remain at risk of relapse even after extended drug-free periods, and well after the acute physical withdrawal symptoms have subsided. Once abstinent, long-lasting negative affective symptoms contribute to increased drug craving, and following exposure to drug-associated cues or stress, an increased risk of relapse which further intensifies over abstinence. New treatments are critically needed to reduce craving and prevent relapse for OUD as current approaches result in ~50% relapse. One promising anti-relapse intervention is kratom, Mitragyna speciosa, a medicinal herb used for centuries in Southeast Asia and more recently in the US, as a self-treatment for pain, depression and anxiety, and opioid withdrawal, craving, and relapse. There are an estimated 3 to 5 million users of kratom in the US, and despite a high prevalence of kratom use as a self-treatment for OUD, its efficacy has yet to be established in humans or animal models. While kratom itself has not yet been examined in the context of addiction, there have been a few studies conducted with alkaloid-rich extracts of kratom and several with mitragynine (MG), its most abundant alkaloid. Findings demonstrate that while both compounds induce positive reinforcing effects, their abuse liability is relatively low. Importantly, as with reports in humans, in animals, kratom (extract) and MG reduce opioid withdrawal symptoms, and these effects are believed to occur via agonistic effects at mu-opioid receptors (MOR). However, unlike classical opioids, MG does not recruit ?-arrestin signaling, and thus the beneficial effects associated with MOR activation (analgesic, anxiolytic, and antidepressant effects) can be obtained with less risk of dangerous side effects (i.e. respiratory depression). Together, these data provide compelling support for the overall objective of this R21 application: to determine in a rat model the potential utility of kratom as an anti- relapse intervention. To do so, we will first determine the composition of commercially-purchased kratom powder with respect to MG as well as 7-hydroxymitragynine (7-HMG), a structurally-related alkaloid that also has agonistic effects at MORs (Aim 1). Although less research has been conducted with 7-HMG since it constitutes significantly less of kratom's total alkaloid content (approximately 2% versus 66% for MG), it has a higher affinity and potency for MORs as compared to MG. Next, using characterized samples, we will determine the efficacy of kratom at reducing relapse vulnerability in response to two of the most common triggers of craving in humans, drug-associated cues (Aim 2) and stress (Aim 3). Effects will be determined in both males and females and correlational analyses will be conducted to determine how the composition of kratom, and specifically, levels of MG and 7-HMG, impacts its efficacy. The question posed in our application is translationally relevant and the results will provide science-based evidence that may influence the scheduling of kratom and help guide its development as a medically-sanctioned treatment for OUD.